1. Field of the Invention
The present invention relates to a prosthesis for the replacement of hard tissues such as human bones or joints having significantly deteriorated or lost functions thereof, and more particularly to a prosthesis having a porous surface structure capable of allowing bone tissues to penetrate so that the support/fixture characteristics of the joining section between the prosthesis and the living tissues can be enhanced. The present invention also relates to a method of making such a prosthesis.
2. Prior Art
As conventional prostheses having porous surface structures capable of allowing bone tissues to penetrate, a plurality of prostheses have been proposed as follows:
(1) A metallic prosthesis having sintered and adhered metallic beads on the surface thereof, as disclosed by U.S. Pat. Nos. 3,855,638 and 4,644,942. PA1 (2) A metallic prosthesis having compressed metallic meshes diffusion-bonded onto the surface thereof by heating at high temperature, as disclosed by European Patent No. 0178650 and U.S. Pat. No. 4,660,755. PA1 (3) A prosthesis having porous metallic sheets secured mechanically to the surface thereof, as disclosed by GB No. 2142830A. PA1 (4) A prosthesis having a porous surface structure with small through holes made by laser processing, as disclosed by U.S. Pat. No. 4,608,052. PA1 (5) A prosthesis having a cast porous component secured to the surface thereof, as disclosed by Japanese Laid-open Patent Application No. 3-123546. PA1 (6) A metallic prosthesis having a surface structure with through holes, the shape of which is almost similar to that of the cancellous bone tissue, as disclosed by Japanese Laid-open Patent Application No. 3-29649. PA1 (7) A prosthesis having a porous lamination component comprising laminated thin sheets, each having through holes provided by punching or etching and a thickness of 150 to 500 .mu.m, made by applying a compression load and heating, or a prosthesis whose surface is partially or entirely coated with the porous lamination component, as disclosed by Japanese Laid-open Patent Application No. 3-49766.
The above-mentioned prostheses, however, have the following problems. The prosthesis (1) has a low volume porosity (the ratio of the volume of pores to the entire volume of the porous component thereof); it is generally said that the typical volume porosity of the above-mentioned conventional prostheses is about 35%. When this volume porosity is low, the relative volume of the bone tissue is small even if the bone tissue completely fills up all pores. Accordingly, the strength of the bonding between the prosthesis and the bone joined thereto is not sufficiently large. In the case of the prosthesis wherein metallic beads are attached to the surface thereof, it is known that the mechanical strength of the prosthesis' base material is significantly lowered by high temperature in the sintering process wherein the beads are attached. According to a report, for example, the fatigue strength of such a prosthesis is lowered to about 1/5 of that of the base material. The sintering process thus significantly adversely affects the durability of the prosthesis when used in the living tissue. In addition, since the bonding strength obtained among the above-mentioned beads is low, the beads may drop after sintering and may be in danger of penetrating articulation surfaces.
In the above-mentioned prosthesis (2), the volume porosity of the porous lamination component thereof is about 50% and the fatigue strength of the porous lamination component is about 70% of the base material thereof, showing a considerable improvement when compared with the above-mentioned prosthesis (1). It is however difficult to control the size and shape of small through holes within desired ranges in the compression process. As a result, the size and shape of the small through holes to be formed are not best suited for the penetration and ingrowth of the bone tissue. Furthermore, the above-mentioned porous lamination component has a disadvantage of generating a great difference in the size and shape of the through holes between those formed in the flat surfaces and those formed in the curved surfaces of the prosthesis because of the difference in the compression load. This changes the degree of the penetration of the bone tissue into the small through holes depending on the portion of the prosthesis, and causes the problem of generating different strength of the bonding between the porous lamination component and the bone to be joined depending on the portion of the prosthesis.
In the case of the prosthesis (3), since the above-mentioned sheets are mechanically bonded to the main body, the sheets cause micro-movements, resulting in wear or melting of the metallic structure thereof, and also resulting in the removal of the sheets in the worst case. This prosthesis is thus not applicable to portions having complicated curved surfaces. In addition, the cost of making the prosthesis is not inexpensive.
The above-mentioned prosthesis (4) has a surface structure having through holes with a diameter of about 300 .mu.m disposed regularly. The through holes however are not open pores communicating with one another but closed pores, thereby preventing bio-liquid from flowing among the bone cells, causing the problem of necrosis at the leading ends of the bone cells.
In the case of the above-mentioned prosthesis (5), since the porous lamination component thereof is made by casting, it is difficult to apply the porous lamination component to portions having complicated curved surfaces. Furthermore, the production cost is high because casting is used.
The above-mentioned prosthesis (6) has a surface structure similar to that of a cancellous bone in size and shape. The size and shape of the through holes in this structure are, however, not best suited for the penetration of bone tissues, thereby causing the problem of preventing bone tissues from sufficiently penetrating the through holes.
In the case of the above-mentioned prosthesis (7), since the thin sheets thereof are as thick as 150 to 500 .mu.m, the porous lamination component thereof cannot be used for complicated curved surfaces or small-diameter cylindrical surfaces. Furthermore, the shape and arrangement of the holes are significantly deformed and dislocated by lamination and compression. It is therefore difficult to properly control the through hole shape best suited for the penetration of bone tissues, thereby causing the problem of preventing bone tissues from sufficiently penetrating the through holes.